1. Field of the Invention
The present invention relates to a method of receiving a session key in a home network and a method of reproducing content using the same, and more particularly to, a method of receiving a session key that makes it easy to administer members of a domain using the session key and a session version of the session key and a method of reproducing content using the same.
2. Description of the Related Art
Digital contents are generally transmitted from a content provider to a user. To protect the unauthorized use of the contents, only an authorized user who paid a content fee can use the digital contents.
Digital contents are encoded using a content key and the authorized user must have the content key in order to prevent unauthorized users from obtaining the contents.
The development of home network technologies makes it possible to move digital contents between user devices. A user who wishes to use digital contents on one or more user devices must pay a fee only once. However, when the contents are moved between user devices in a manner of reproduction, an unauthorized user can obtain and use the contents. Hence, a home network technology capable of moving digital contents between user devices in a user home network and preventing an unauthorized user from obtaining and using the contents are required.
In particular, with respect to high definition (HD) contents in U.S. digital broadcasting, the U.S. Federal Communication Commission has required from July 2005 a technology that adds a 1-bit broadcast flag (BF) in order to prevent an unauthorized user from using the contents. Digital broadcasting contents between a home server and user devices must be securely used.
FIG. 1 is a diagram illustrating a structure of a home network. Referring to FIG. 1, a content provider (CP) transmits content to a home server (HS) through a transmission channel 10.
The HS is connected to user devices D1, D2, D3, D4, administers a subscription and a withdrawal of an authorized user device, and transmits the contents to authorized user devices, i.e., joined devices.
A domain is a group of user devices connected to one HS. The domain changes whenever new user devices join to, or older user devices withdraw from, the home network. A domain administration method using a session key is used to prevent a withdrawn user device from obtaining the content.
FIG. 2 is a flowchart illustrating a conventional domain administration method. Referring to FIG. 2, the HS transmits encoded content E K_cont, Cont that is created by encoding content Con using a predetermined content key K_cont, and an encoded content key E K_sess, K_cont that is generated by encoding the content key K_cont using a predetermined session key K_sess to current user devices D1, D2, D3, D4 (Operation 210). The current user devices are referred to as authorized devices that have joined to and are connected to the HS. The encoded content key E K_Sess, K_cont is stored in each of the user devices D1, D2, D3, D4 in a file format of a license.
The HS transmits the session key K_sess used to encode the content key to the user devices D1, D2, D3, D4 through a secure transmission route (Operation 220).
The user devices D1, D2, D3, D4 decode the encoded content key E K_sess, K_cont using the session key K_sess to obtain the content key K_cont, and decode the encoded content key E K_cont, Cont using the content key K_cont to obtain and reproduce content Cont (Operation 230).
Suppose that the user device D4 withdraws from the home network (Operation 240).
The HS generates a new session key K_sess' (Operation 250) and transmits the generated session key K_sess' to the user devices D1, D2, D3 (Operation 260).
The user devices D1, D2, D3 encode the content key K_cont using the session key K_sess', generate a new encoded content key E K_sess', K_cont, and store the encoded content key E K_sess', K_cont in the license file (Operation 270). The user devices D1, D2, D3 renew the license file.
The user devices D1, D2, D3 decode the encoded content key E K_sess', K_cont using the new session key K_sess' to obtain the content key K_cont, and decode the encoded content key E K_cont, Cont using the content key K_cont to obtain and reproduce content Cont (Operation 280).
New content Cont' is transmitted to the user devices D1, D2, D3 using the new session key K_sess'. The HS transmits encoded content E K_cont', Cont' created by encoding new content Cont' using a predetermined new content key K_cont', and an encoded content key E K_sess', K_cont' that is generated by encoding the content key K_cont' using the new session key K_sess' to the current user devices D1, D2, D3.
The user device D4 cannot obtain the content key K_cont' for newly transmitted content Cont' since it does not have the new session key K_sess'. Therefore, the user device D4 cannot reproduce content distributed in the home network after withdrawing from the home network.
According to the conventional method, the user device D4 cannot receive and reproduce content Cont transmitted to each of the user devices before withdrawing from the home network.
When the user device D4 does not have the content key K_cont, the user device D4 can obtain the content key K_cont by receiving the license file including the encoded content key E K_sess, K_cont. However, after withdrawal of the user device D4, since the user devices D1, D2, D3 renew the license, the user device D4 does not have the encoded content key E K_sess, K_cont using the session key K_sess any more, thereby failing to obtain the content key K_cont.
When the user device D4 loses the content E K_cont, Cont and the content key K_cont, although the user device D4 can use content Cont that is content before withdrawing from the home network, the user device D4 cannot use the content E K_cont, Cont transmitted from the user devices D1, D2, D3.